Organize examples into subdirectories

[shiffman]

At present we have

javascript
  --> example1
  --> example2
p5js
  --> example1
  --> example2

with the idea that we'll also eventually have:

threejs
  --> example1
  --> example2
etc.

At the moment the p5js directory has many examples, maybe change to. . .

p5js
  --> images
    --> example1
    --> example2
  --> sound
    --> example1
    --> example2
  --> text
    --> example1
    --> example2

?

[cvalenzuela]

Closed by #56. gracias @montoyamoraga!